Abstract: Pseudo-Time Lapse Borehole Seismic Imaging of Fluid Bearing Fractures

DYER, B.C, T. WALLROTH, P. JAQUES, and A.J. JUPE

A borehole seismic imaging system has been developed by CSMA consisting of a borehole sparker seismic source, string of hydrophone receivers and tomographic transmission/reflection velocity and wavefield imaging software. The system was employed in a crosshole demonstration survey at the Chalmers Hot Dry Rock research site at Fjallbacka, Sweden, to illustrate the technical and practical viability of the system in high resolution time-lapse imaging of fluid bearing fracture zones. Surveys were performed under ambient pressure conditions and at an elevated downhole pressure of 3.35 MPa over a well characterized fracture zone to image the distribution of fractures opened by the pressurization.

P wave traveltimes were picked from the raw data from which velocity tomograms were formed and the trace data were migrated using a pre-stack diffraction stack technique. Integration of log and hydraulic test results with the tomograms and wavefield images enabled the spatial distribution of the different lithologies and fracture zones between the wells to be interpreted. Difference (pseudo-time lapse) images were formed by subtracting the unpressurized and pressurized tomograms and wavefield images. These images were used to identify fracture zones opened by the pressurization. From the difference images it was found that the flowing fracture zones at the two wells are poorly connected in the interwell region. This interpretation is consistent with the results of interference tests between the wells which found a high impedance connection.

In view of the limited availability of boreholes suitable for crosshole work in oil field applications, CSMA are currently developing the sparker and hydrophone system to operate in a singlewell configuration.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria